Generally there are two commercial types of evaporation-condensation refrigeration processes known in the art, namely, mechanical refrigeration and absorption refrigeration. In mechanical refrigeration, the low pressure vapor from the evaporator is compressed by a mechanical compressor to the high pressure necessary for condensation into the liquid state by the condenser. In absorption refrigeration, the refrigerant vapor is withdrawn from the evaporator by absorption in a liquid absorbent which is then pumped to a separator or a regenerator where the absorbent is heated to desorb the refrigerant and produce the refrigerant vapor feed for condensation in the condenser. In one prior art absorption refrigeration process there is employed a highly volatile compound, such as hydrogen, producing partial vapor pressures inversely related to the partial vapor pressures of the refrigerant, such as ammonia, in the absorber and regenerator to thus produce about the same total vapor pressure in the absorber and regenerator to eliminate the need for a mechanical pump to feed the absorbent liquid to the regenerator; the total high vapor pressure in the evaporator does not interfere with evaporation of the refrigerant since the partial vapor pressure of the refrigerant in the evaporator is low.
Mechanical and absorption refrigeration processes are utilized in many commercial applications. Absorption refrigeration is particularly attractive when low level waste heat is available for heating the absorbent liquid in desorption of the refrigerant. Mechanical refrigeration is generally preferred for obtaining lower temperatures; a single mechanical refrigeration cycle using propylene refrigerant can practically achieve a -50.degree. F. (-45.degree. C.) temperature, whereas the practical limit for an absorption refrigeration cycle employing ammonia is about -10.degree. F. (-23.degree. C.), although temperatures of -50.degree. F. (-45.degree. C.) are obtainable by increasing the regeneration or desorption temperature from 220.degree. F. (104.degree. C.) to 265.degree. F. (130.degree. C.). Lower refrigeration levels can be obtained using evaporators operating under vacuum; however, this is neither practical nor energy efficient. Below about -50.degree. F. (45.degree. C.), a cascade system, such as a first stage mechanical refrigeration cycle employing propylene to produce a temperature of about -50.degree. F. (-45.degree. C.) and a second mechanical refrigeration stage employing ethylene to produce the temperature below -50.degree. F., is used. In a cascade system, the heat exchange between the stages requires a temperature difference, usually about 10.degree. F. (6.degree. C.) to condense the lower stage refrigerant, thus producing a loss in efficiency.